1. Field of the Invention
The present invention relates to a semiconductor device having a TFT (thin film transistor) provided on an insulating substrate such as glass, and a method of manufacturing the semiconductor device.
2. Discussion of the Related Art
As the semiconductor having the TFT formed on the insulating substrate made of glass or the like, there have been known an active liquid crystal display device, an image sensor and the like, which use the TFT for driving a pixel.
A thin film silicon semiconductor is generally used for the TFT used in these devices. The thin film silicon semiconductor is roughly classified into the amorphous silicon semiconductor (a-Si) type and the crystalline silicon semiconductor type. The amorphous silicon semiconductor is most generally used because the manufacturing temperature is low, it can be relatively readily manufactured by a vapor phase method, and the mass productivity is sufficient. However, since the physical properties of the amorphous silicon semiconductor is inferior to the crystalline silicon semiconductor such as the electrical conductivity or the like, there is a strong demand to establish a method for manufacturing the TFT formed of the crystalline silicon semiconductor for the purpose of obtaining the higher speed characteristics in the future. As the crystalline silicon semiconductor, there have been known non-single crystalline silicon semiconductors such as polycrystalline silicon, microcrystalline silicon, amorphous silicon containing crystal components, semi-amorphous silicon having an intermediate state between the crystal property and the amorphous property, and the like. Hereinafter, the non-single crystalline silicon semiconductors having these crystal properties are called a crystalline silicon.
As a method of obtaining the thin film silicon semiconductors with these crystal properties, there have been known the following methods.
(1) A crystalline film is directly formed at the time of film formation.
(2) The energy of a laser illumination is applied to an amorphous semiconductor film which has been previously formed to provide the crystal property.
(3) A heat energy is applied to an amorphous semiconductor film which has been previously formed to provide the crystal property.
However, in the method (1), it is technically difficult to uniformly form a film having the excellent semiconductor physical properties all over the upper surface of a substrate. Further, since the film forming temperature is high, that is, 600.degree. C. or more, there rises such a problem in costs that an inexpensive glass substrate cannot be used. In the method (2), in the case of an example of an excimer laser which is most generally used now, there rises a problem that a through-put is low because a laser beam radiated area is small. Furthermore, the stability of the laser beam is insufficient to uniformly treat the entire upper surface of a large-area substrate, whereby it strongly seems as if this method is the technique for the coming generation. In the method (3), there is an advantage in that this method can cope with the large-area of the substrate in comparison with the methods (1) and (2). However, a high temperature of 600.degree. C. or more is required as a heating temperature, and taking the inexpensive glass substrate used into consideration, it is necessary to further decrease the heating temperature. In particular, the current liquid-crystal display unit advances to a large screen, and for that reason, it is necessary to use a large-scale glass substrate likewise. When such a large-scale glass substrate is used, there rises a serious problem that the contraction or strain of the substrate in the heating process essential to the semiconductor manufacture makes the accuracy in mask matching or the like deteriorate. In particular, in the case of the 7059 glass which is most generally used now, the temperature of the strain point is 593.degree. C., whereby the conventional heat crystallization method causes the substrate to be largely deformed. Moreover, in addition to the temperature problem, since the current process requires heating time of several tens hour or more necessary for crystallization, it is also necessary to shorten the heating time.